1. Field of the Invention
The present invention broadly relates to tracking a wideband, frequency-modulated RF signal and, more particularly, is concerned with an automatic frequency-tracking circuit that employs an improved phase-locked loop incorporating means which compensate for frequency drift in the RF signal so as to maintain the loop locked on the signal.
2. Description of the Prior Art
When an RF carrier signal is frequency modulated (FM), its frequency increases and decreases, above and below the original carrier signal frequency called the center frequency, in accordance with the amplitude variations of a modulating signal. Thus, during the process of frequency modulation, new frequencies, called sideband frequencies, are produced above and below the unmodulated carrier signal's center frequency. The sideband frequencies are multiples of the frequency of the modulating signal. It is these sideband frequencies that contain the signal intelligence and combine with the unmodulated carrier signal to produce the FM signal. The bandwidth of an FM signal is the frequency range between only the extreme upper and extreme lower sideband frequencies whose amplitudes are viewed as significant with respect to intelligence-carrying capacity; however, even these sideband frequencies can extend far from the center frequency of the FM signal. In tracking the frequency of the FM signal for recovery or extraction of the intelligence (i.e., the modulating signal) from the FM signal, devices such as demodulators (or detectors) are utilized. FM demodulators frequently utilize phase-locked loops.
The phase-locked loop basically includes three components: a phase detector, a loop amplifier and a voltage controlled oscillator (VCO). The phase detector receives the FM signal and a local signal generated by the loop VCO, compares their phases and generates a DC voltage which is a measure of the phase difference between the two signals, the phase difference being produced by the difference between the frequencies of the FM signal and the VCO generated signal as well as by inherent loop lag. The DC voltage is filtered and amplified by the loop amplifier and then applied to the VCO so as to drive the signal generated by the VCO in a direction that reduces the phase difference (and thus the frequency difference) between the FM signal and the VCO generated signal. When the loop is "locked," the DC voltage applied to the VCO is such that the frequency of the VCO generated signal is equal to the average frequency of the FM signal, that being, the center frequency of the unmodulated carrier signal portion of the FM signal. The filtered and amplified DC voltage, therefore, represents the intelligence extracted from the FM signal, i.e., the original frequency modulating signal. The modulating signal may be a varying video voltage which is utilized by other circuitry coupled to the FM demodulator to produce a TV picture.
The frequency of an FM signal, as it is being applied to the loop phase detector, will be continually shifting between the upper and lower extreme sidebands of the FM signal while the VCO generated signal, also being applied to the loop phase detector, will try to remain constant, that is, equal to the average frequency of the FM signal, that being the center frequency of the unmodulated carrier signal portion of the FM signal. However, the phase detector will be outputting the phase difference between the signals which is a DC voltage continually moving between positive and negative amplitudes reflecting the continual shifting of the FM signal frequency between its extreme upper and lower sidebands relative to the comparatively constant frequency of the VCO generated signal. The fluctuating DC voltage is amplified and applied to the VCO so as to drive its output signal in a direction which tends to reduce the amplitude of the phase difference output of the phase detector. Thus, the DC voltage applied to the VCO tends to bias it so as to pull it on frequency.
In the case where the FM signal being demodulated by the loop has a very wide bandwidth, called wideband FM, the frequency shifting is so extreme that oftentimes the DC output of the phase detector, which is applied to the loop amplifier to drive the VCO to shift the frequency of its output signal correspondingly, has such wide amplitude swings that the loop amplifier is no longer optimized in its capability to swing positive and negative to follow the input frequency shifts. Consequently, the intelligence begins to distort when the limits of the loop amplifier are exceeded. This is a design limitation of amplifiers known in the current state of the art. As the amplifier's limits are exceeded, it changes from linear operation to nonlinear operation at which time the intelligence represented by its output becomes distorted, i.e., is no longer linearly proportional to the input to the amplifier. Not only is the linearity of the amplified voltage lost when the amplifier's limits are exceeded, but a point is reached where lock of the VCO generated signal on the FM signal is actually broken or lost and the loop VCO begins to skip cycles.
Also, in the case where the average frequency of the FM signal being demodulated drifts off from its center frequency due to, for instance, transmitter drift, the average level of the DC voltage produced by the loop detector will follow the frequency drift. This results in imbalance of the amplifier which decreases its effective range of linear operation. Thus, in the presence of frequency drift of the transmitted signal, the modulation swing of the system must be reduced in order to maintain an undistorted demodulated signal at the receiver end of the system.